This invention is in the field of true north reference devices, preferably of the self-leveling gyroscopic type.
Numerous types of north-seeking gyrocompasses are known in the art. One type of north-seeking gyrocompass is a pendulous gyroscope having its spin axis oriented horizontally. This type of gyroscope tends to precess under the rotation of the earth to align its spin axis with true north. In such a mechanization, the gyroscope is allowed to oscillate back and forth across a north-south meridian. However, the precision with which the direction of true north may be obtained from such a device is limited by error torques, such as friction in the suspension of the gyroscope, which torque the gyroscope about the vertical axis and hold the gyroscope off north. The magnitudes of these torques cannot be easily determined and in many applications are usually assumed to be constant. Also, because of the oscillations about true north inherent in this type of mechanization, the minimum period of time during which an indication of true north can be obtained is limited to the period of at least one oscillation. The length of this period is also dependent on the initial misalignment of the spin axis with north.
Another type of north-seeking gyrocompassing system is described in U.S. Pat. No. 3,222,795. The device disclosed therein includes an inertial platform having three gyroscopes. The platform is slaved to local horizontal orientation by erection servo-mechanisms. Also, the sensitive axis of one of the gyroscopes must be initially aligned in an approximately east-west direction by using a compass or the like. The requirement for an inertial platform equipped with gyroscopes and servomechanisms and associated electronics, and also the requirement for an external compass would contribute greatly both to the cost and to the size of this device.